Compact Objective Lens with Enhanced Distortion for Near-Infrared Imaging

ABSTRACT

The current invention describes a compact objective lens with enhanced distortion for near-infrared imaging, comprising a positively powered, aspheric, and meniscus first lens element; a negatively powered, aspheric, and meniscus second lens element; a positively powered, aspheric, and biconvex third lens element; a negatively powered, aspheric, and meniscus fourth lens element; a negatively powered, aspheric, and meniscus field corrector element; and a detector assembly comprising a window and a detector plane where the light rays come to focus.

GOVERNMENT INTEREST

The invention described herein may be manufactured, used, sold,imported, and/or licensed by or for the Government of the United Statesof America.

FIELD OF THE INVENTION

This invention is applicable to the field of optics for infraredimaging, particularly in regards to an objective lens for near-infraredimaging.

BACKGROUND OF THE INVENTION

Commercial cell phone products employ extremely complex objective lensassemblies in order to provide high quality imagery within a very smallvolume. The cell phone camera lens assembly typically consists of threeto six individual lens elements, of which most have complexnon-spherical surface shapes that correct geometric aberrations over alarge field of view and relatively fast F # value. Generally speaking,the faster (i.e., lower value) of the F #, the better the image qualityunder low light conditions. For example, U.S. Pat. No. 8,179,615describes several embodiments of the current art of a cellphone cameralens having fields of view ranging from ±30.6° to ±40.2° and F #'svarying between 2.20 and 2.60. These embodiments are all optimized forbest performance over the visible wavelength spectrum from 486.1 nm to656.3 nm. They also provide very low optical distortion, generally noworse than ±3%. The individual elements are made from optical plasticmaterials which are capable of being molded in mass quantities to reducecost. The physical length of these assemblies are on the order of about4.8 mm and utilize an image format of roughly ±4.5 mm with 1280×720pixels and a 0.0035 mm pitch (a typical size for a cell phone CMOSimager.) The commercial cell phone camera lenses are not ideal for nightimaging however, where there is a need for imaging over thenear-infrared spectrum from 650 nm to 850 nm, along with more difficultrequirements for both faster F #'s, on the order of 1.35 or lower, aswell as larger focal plane format pixel sizes, on the order 0.010 mmwhich have an increased light gathering area compared to smallercommercial pixel pitches. Simple linear scaling of the prior art formsfor the larger pixel size is not sufficient to provide faster F #'s withnear diffraction-limited image quality in the near-infrared spectrum. Itis well known in the art of optical design that while a design optimizedfor a fast F # may be “stopped down” for operation at a slower F # andstill maintain a given geometric image quality, the reverse of thissituation is not true.

SUMMARY OF THE INVENTION

The current invention describes an objective lens assembly opticaldesign optimized for use in near-infrared night imaging applications.The significant performance characteristics include maintainingnear-diffraction limited image resolution over a wide ±32.5° field ofview while operating in the near-infrared spectrum from 650 nm to 850 nmand having a fast F # of at least 1.44.

An exemplary compact objective lens with enhanced distortion fornear-infrared imaging comprises a positively powered, aspheric, andmeniscus first lens element; a negatively powered, aspheric, andmeniscus second lens element; a positively powered, aspheric, andbiconvex third lens element; a negatively powered, aspheric, andmeniscus fourth lens element; a negatively powered, aspheric, andmeniscus field corrector element; and a detector assembly comprising awindow and a detector plane where the light rays come to focus.

BRIEF DESCRIPTION OF THE DRAWINGS

Additional advantages and features will become apparent as the subjectinvention becomes better understood by reference to the followingdetailed description when considered in conjunction with theaccompanying drawings wherein:

FIG. 1 shows an exemplary arrangement of compact objective lens withenhanced distortion for near-infrared imaging with ray traces;

FIG. 2 shows a plot of an exemplary Modulation Transfer Function of thelens;

FIG. 3 shows a plot of an exemplary geometric distortion of a lensimage;

FIG. 4 shows a plot of exemplary beam footprints for several fieldpositions as seen in the plane of an air gap sitting 0.7 mm in front ofthe first lens;

FIG. 5 shows a lens prescription listing with exemplary parameters ofradius of curvature, thickness, index of refraction, semi-diameter,conic constant, and aspheric coefficients;

FIG. 6 illustrates a table comparing critical differences between theprior art and the current invention in terms of paraxial lens shapefactors; and

FIG. 7 illustrates a table demonstrating additional differences betweenthe disclosure of U.S. Pat. No. 8,179,615B1 and the current design.

DETAILED DESCRIPTION

The current invention describes an objective lens assembly opticaldesign optimized for use in near-infrared night imaging applications.The significant performance characteristics include maintainingnear-diffraction limited image resolution over a wide ±32.5° field ofview while operating in the near-infrared spectrum from 650 nm to 850 nmand having a fast F # of at least 1.44. The length of the opticalassembly is approximately 17.8 mm and the format size is a larger ±6.4mm to accommodate larger 0.010 mm pixel sizes in a 1280×720 elementarray. An additional feature of this lens design which is different fromthe disclosure of U.S. patent application Ser. No. 16/574,498, is thatthe design utilizes optical distortion in order to improve theresolution near the center of the field of view. By deliberatelyimplementing negative “barrel” distortion into the system, the axialfocal length of the system can increase while keeping the field of viewconsistent for a given focal plane format dimension. This systemcomprises a positively powered, aspheric, and meniscus first lenselement; a negatively powered, aspheric, and meniscus second lenselement; a positively powered, aspheric, and biconvex third lenselement; a negatively powered, aspheric, and meniscus fourth lenselement; a negatively powered, aspheric, and meniscus field correctorelement; and a detector assembly comprising a window and a detectorplane where the light rays come to focus. The meniscus field correctoris the element that introduces a majority of the distortion in thesystem. It may be noted that the definitions of lens shape factors(meniscus, bi-convex, bi-concave, etc.) are determined by the paraxiallens curvatures that are best emphasized very close the optical axis,and can be quickly determined by inspection of the base radii ofcurvature terms (r²), notwithstanding the higher order aspheric terms.The invention is best understood by referencing the ray trace drawingshown in FIG. 1.

Referencing the ray trace in FIG. 1, light ray bundles from a scene forthe central field of view 1 and ray bundles for the edges of the field2, and all fields between, enter through the first lens element 3 whichis meniscus, aspheric, and has positive optical power. This lens maypreferably be made of a material such as the Cyclic Olefin Copolymer(COC), with trade names such as “Topas” sold by Topas Advanced PolymersGmbH, having an index Nd=1.5337 and dispersion Vd=56.288. It has aparaxial optical power of approximately +77.7 diopters. The light raysthen enter the second lens element 4 which is of a general meniscusshape, i.e. having a concave surface on one side and a convex surface onthe other, along with aspherics to provide a net negative optical power.This lens is preferably made of a material such as AL-6263-(OKP4HT),which is sold commercially by AngstromLink, with an index Nd=1.6319 anddispersion Vd=23.328. It has a paraxial optical power of about −102.7diopters. The light rays then pass through the third element 5 which isgenerally bi-convex with aspheres to provide positive optical power.This lens is preferably made of COC material. It has an optical power ofapproximately +126.5 diopters. Light then enters the negatively poweredfourth optical element 6 which is meniscus in general form, and alsoutilizes aspheric curvatures. This lens element is preferably made froma Cyclic Olefin Polymer (COP) such as the trade name “E48R” from ZeonCorp. It has an index Nd=1.5312 and dispersion Vd=56.044. It has anoptical power of −5.06 diopters. The light rays then enter the negativefield corrector lens 7 which is generally of meniscus shape with strong,high order aspheric curvatures. This lens is preferably made from COCmaterial. It has an optical power of −69.9 diopters. A flat glass windowor filter element 8 is then included in the optical path as it may bepart of the detector assembly which in turn also provides the detectorpixel locations in the plane 9 where the image is focused. The totaloptical physical length is about 17.2 mm, and the total mass of theplastic elements and the glass window is on the order of 0.95 grams.

FIG. 2 shows a plot of the Modulation Transfer Function of the lens,which is a comprehensive measure of the resolution of the lens image.The top curve 10 shows the diffraction limit, which is the MTF of atheoretically perfect lens with no geometric aberrations. The curvesbelow 10 are representative of the design itself, which is very close tothe diffraction limit over most of the field of view as measured out to50 cycles per millimeter in image space, the Nyquist sampling cutofffrequency of a detector with 0.010 mm pitch.

FIG. 3 shows a plot of the geometric distortion of the lens image. Thegrid of continuous lines shows the ideal case of 0% distortion, and thearray of dots mark the actual field positions as mapped through thelens. The farthest dot mark 11 in the corner corresponds to a maximumdistortion of −6.92%. This distortion value enables the optical system'seffective focal length (EFL) to be longer by 6.92% while still providingthe same total field of view onto the focal plane detector format. Inthe preferred embodiment, the EFL is 10.8 mm. The longer focal lengththen provides a narrower instantaneous field of view (IFOV) for thepixels in the center, which in turn provides more pixels on a target ofa given size. Normally this level of optical distortion would beobjectionable if presented directly to the human eye, therefore thepreferred implementation of this technique is to use this lens within animage projecting system which can provide either an electronic digitalimage correction in a video output signal going to a display, or acompensating distortion in a magnifying eyepiece which views a display,such that the human eye would perceive a net distortion of 0.00%.

FIG. 4 shows a plot of the beam footprints for several field positionsas seen in the plane of an air gap sitting 0.7 mm in front of the firstlens 3. The largest footprint 12 corresponds to the on-axis field raybundle, indicating that this field is operating at the paraxial F # of1.45. Subsequent fields show gradually increasing vignetting, finallyresulting in the footprint 13 which corresponds to the farthest edge ofthe field of view. This farthest field position has about 61.2% of thelight relative to the on-axis ray bundle. The use of vignetting toobtain a very fast axial F # is explained in the reference, OE Magazine2002, “Tricks of the Trade”, and is a significant difference from theprior art exhibited by U.S. Pat. No. 8,179,615.

FIG. 5 shows the lens prescription listing with the parameters of radiusof curvature, thickness, index of refraction, semi-diameter, conicconstant, and aspheric coefficients. The sag of the lens curvatures isdefined by the following equation where “z”=sag, “c”=spherical radius ofcurvature, “k”=conic constant, “r” is the position along thesemi-diameter, and the coefficients “α_(x)” correspond to theeven-ordered aspheric polynomial terms:

z=[(cr ²)÷(1+SQRT(1−(1+k)c ² r ²))]+α₁ r ⁴+α₂ r ⁶+α₃ r ⁸+α₄ r ¹⁰+α₅ r¹²+α₆ r ¹⁴+α₇ r ¹⁶

Materials with an index=1.000 are air gaps, and all values within thetable assume a nominal temperature of 20° C. and a pressure of 1atmosphere.

Novelty as Compared to the Prior Art. The table in FIG. 6 illustratesthe critical differences between the prior art and the current inventionin terms of paraxial lens shape factors.

The principle novelty may thus be summarized by the followingcharacteristics:

-   -   Near-infrared spectral band.    -   Fast F # of at least 1.44.    -   Use of vignetting for the edges of field of view.    -   Shape factor arrangement of the five powered lens elements (see        table).    -   Deliberate use of negative “barrel” optical distortion to        achieve a longer focal length and hence better resolution of a        target object, while maintaining the same field of view for a        given focal plane format size.

The next table in FIG. 7 demonstrates additional differences between thedisclosure of U.S. Pat. No. 8,179,615B1 and the current design, whereinthe variables illustrated in FIG. 7 are defined as follows:

Variable Definition CT₄ Thickness of the fourth optical element in thesystem. CT₅ Thickness of the fifth optical element in the system. fEffective focal length of the entire system. f₁ Focal length of thefirst optical element in the system. T₄₅ Distance along the optical axisbetween the 4th and 5th elements in the system.

The differences in shape factor from application Ser. No. 16/574,498 andthe distinct contrast between the disclosures of U.S. Pat. No.8,179,615-B1 and U.S. Pat. No. 8,934,179 versus the current inventiondemonstrate that the claimed invention is not taught by prior art.

It is obvious that many modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as described.

What is claimed is:
 1. A compact objective lens assembly with enhanceddistortion for near-infrared imaging, comprising: an aspheric andmeniscus first lens element first disposed along an optical path toprovide a positive optical power; a meniscus second lens elementdisposed next along the optical path to provide a negative opticalpower; a biconvex third lens element with aspheres disposed next alongthe optical path to provide a positive optical power; a meniscus fourthlens element having aspheric curvatures disposed next along the opticalpath to provide a negative optical power; a negative field correctorlens which is of meniscus shape with strong, high order asphericcurvatures disposed next along the optical path; and a windowed detectorassembly disposed next along the optical path to focus light raysentering the windowed detector assembly to a detector plane, wherein animage is focused onto the detector plane for near-infrared imaging. 2.The compact objective lens assembly according to claim 1, wherein thecompact objective lens assembly has a length of approximately 17.8 mm;and its format size is a larger ±6.4 mm to accommodate larger 0.010 mmpixel sizes in a 1280×720 element array.
 3. The compact objective lensassembly according to claim 1, wherein the aspheric and meniscus firstlens element is based on a Cyclic Olefin Copolymer lens material.
 4. Thecompact objective lens assembly according to claim 1, wherein theaspheric and meniscus first lens element is characterized by an indexNd=1.5337; dispersion Vd=56.288; and a paraxial optical power ofapproximately +77.7 diopters.
 5. The compact objective lens assemblyaccording to claim 1, wherein the meniscus second lens element has aconcave surface on one side and a convex surface on another side, alongwith aspherics to provide a net negative optical power.
 6. The compactobjective lens assembly according to claim 1, wherein the meniscussecond lens element is characterized by an index Nd=1.6319; dispersionVd=23.328; and a paraxial optical power of about −102.7 diopters.
 7. Thecompact objective lens assembly according to claim 1, wherein thebiconvex third lens element with aspheres is based on a Cyclic OlefinCopolymer lens material.
 8. The compact objective lens assemblyaccording to claim 1, wherein the biconvex third lens element withaspheres has an optical power of approximately +126.5 diopters.
 9. Thecompact objective lens assembly according to claim 1, wherein themeniscus fourth lens element having aspheric curvatures is based on aCyclic Olefin Polymer lens material.
 10. The compact objective lensassembly according to claim 1, wherein the meniscus fourth lens elementhaving aspheric curvatures is characterized by an index Nd=1.5312;dispersion Vd=56.044; and an optical power of −5.06 diopters.
 11. Thecompact objective lens assembly according to claim 1, wherein thenegative field corrector lens is based on a Cyclic Olefin Copolymer lensmaterial.
 12. The compact objective lens assembly according to claim 1,wherein the negative field corrector lens has an optical power of −69.9diopters.
 13. The compact objective lens assembly according to claim 1,wherein the windowed detector assembly has an optical window throughwhich an image is focused along the optical path onto the detector planehaving detector pixel locations for near-infrared imaging.
 14. Thecompact objective lens assembly according to claim 13, wherein saidoptical window is either a flat glass window or a filter element. 15.The compact objective lens assembly according to claim 1, wherein atotal optical physical length is about 17.2 mm, and a total mass isabout 0.95 grams.
 16. A method of near-infrared imaging using thecompact objective lens assembly with enhanced distortion according toclaim 1, the steps of the method of near-infrared imaging comprising:light ray bundles from a scene enter through the aspheric and meniscusfirst lens element to provide a positive optical power to its output offirst light rays; the first light rays then enter the meniscus secondlens element to provide a negative optical power to its output of secondlight rays; the second light rays then pass through the biconvex thirdlens element with aspheres to provide positive optical power to itsoutput of third light rays; the third light rays then enter the meniscusfourth lens element having aspheric curvatures to provide a negativeoptical power to its output of fourth light rays; the fourth light raysthen enter the negative field corrector lens which is of meniscus shapewith strong, high order aspheric curvatures to produce field correctedlight rays; and said field corrected light rays pass through an opticalwindow disposed along the optical path to be focused on detector pixellocations as a focused image.
 17. The method of near-infrared imagingaccording to claim 16, wherein said light ray bundles from a scene arecomprised of light ray bundles from a scene for the central field ofview, light ray bundles for edges of the field, and light ray bundlesfrom all fields between.
 18. The method of near-infrared imagingaccording to claim 16, wherein said meniscus second lens element has aconcave surface on one side and a convex surface on the other, alongwith aspherics to provide a net negative optical power to its secondlight rays.
 19. The method of near-infrared imaging according to claim16, wherein said optical window disposed along the optical path is aflat glass window or filter element disposed in said optical path as apart of its detector assembly having said detector pixel locations in aplane to which said image is focused.
 20. The method of near-infraredimaging according to claim 16, wherein its near-diffraction limitedimage resolution is achieved over a wide ±32.5° field of view whileoperating in a near-infrared spectrum from 650 nm to 850 nm and having afast F # of at least 1.44.